The invention relates to power transmission mechanisms, and more particularly to a steering mechanism for power transmission mechanisms of the type having two output shafts adapted to be connected to respective propulsion means disposed at opposite sides of a vehicle.
Throughout this specification, the expression "propulsion means" is intended to include wheels or tracks, and the expression "fixed to" is intended to mean "integral with or rigidly secured on".
It is known to steer a vehicle having a power transmission mechanism of the type referred to by varying the relative rotational speeds of the output shafts by an amount which is independent of the gear ratio being transmitted by the power transmission mechanism. This gives a larger turning circle in the higher gear ratios, which is in accord with the requirements of the vehicle for stable steering conditions.
In one known arrangement for effecting this, the output shafts are driven by respective epicyclic gears. The variable-ratio input member of each epicyclic gear is an annular gear, the output member is a planet carrier, and there is a negative auxiliary drive to a sun gear from one of two half-shafts which are driven by a differential gear from the input side of the power transmission mechanism. Each sun gear can be brought to rest by applying an associated brake, so that when either one of the sun gears and its associated half-shaft are braked the differential gear operates to double the rotational speed of the other half-shaft and its associated sun gear. This causes the relative rotational speeds of the planet carriers and the associated output shafts to vary, and thus effects steering of the vehicle. When both brakes are in the released condition, both half-shafts, both sun gears and thus both output shafts rotate at the same speed, so that the vehicle moves straight ahead.
In another known arrangement for varying the relative rotational speeds of the output shafts, said shafts are driven by respective epicyclic gears as described in the preceding paragraph, but there are no brakes associated with the sun gears, and the differential gear and half-shafts are replaced by a single shaft driveably connected to both sun gears by odd and even numbers of gears respectively. This applies equal and opposite reaction forces to the sun gears, and assuming that the drive resistance is the same at each wheel or track the vehicle moves straight ahead when there is no rotation of the sun gears and the shaft driveably connected thereto. For this reason said shaft has become known as a "null" shaft. Said shaft is driveable in either direction by hydrostatic means comprising a fixed capacity hydraulic motor supplied with oil under pressure by a variable and reversible delivery hydraulic pump driven from the input side of the power transmission mechanism. Rotation of the null shaft by the hydrostatic means causes the sun gears to rotate in opposite directions, as a result of which the relative rotational speeds of the planet carriers and the associated output shafts vary to effect steering of the vehicle. However, it has been found in practice that the hydrostatic means do not always function reliably.
The object of the present invention is to provide a novel arrangement for varying the relative rotational speeds of the output shafts which utilises a null shaft but does not employ associated hydrostatic means.